ZubaxGNSS compilation broken on autotest

[peterbarker]

Bug report

BB-WAF: In file included from /mnt/volume_nyc3_01/autotest/APM/APM/libraries/AP_Baro/AP_Baro.h:4:0,
BB-WAF:                  from /mnt/volume_nyc3_01/autotest/APM/APM/libraries/AP_Vehicle/AP_Vehicle.h:24,
BB-WAF:                  from /mnt/volume_nyc3_01/autotest/APM/APM/libraries/AP_Compass/AP_Compass.cpp:5:
BB-WAF: /mnt/volume_nyc3_01/autotest/APM/APM/libraries/AP_Compass/AP_Compass.cpp:247:67: error: index 2 denotes an offset greater than size of 'const Compass::mag_state [2]' [-Werror=array-bounds]
BB-WAF:      AP_GROUPINFO("OFS3",    13, Compass, _state._priv_instance[2].offset, 0),
BB-WAF:                                                                    ^
BB-WAF: /mnt/volume_nyc3_01/autotest/APM/APM/libraries/AP_Param/AP_Param.h:94:71: note: in definition of macro 'AP_VAROFFSET'
BB-WAF:  #define AP_VAROFFSET(type, element) (((ptrdiff_t)(&((const type *)1)->element))-1)
BB-WAF:                                                                        ^~~~~~~
BB-WAF: /mnt/volume_nyc3_01/autotest/APM/APM/libraries/AP_Param/AP_Param.h:109:54: note: in expansion of macro 'AP_GROUPINFO_FLAGS'
BB-WAF:  #define AP_GROUPINFO(name, idx, clazz, element, def) AP_GROUPINFO_FLAGS(name, idx, clazz, element, def, 0)
BB-WAF:                                                       ^~~~~~~~~~~~~~~~~~
BB-WAF: /mnt/volume_nyc3_01/autotest/APM/APM/libraries/AP_Compass/AP_Compass.cpp:247:5: note: in expansion of macro 'AP_GROUPINFO'
BB-WAF:      AP_GROUPINFO("OFS3",    13, Compass, _state._priv_instance[2].offset, 0),
BB-WAF:      ^
BB-WAF: compilation terminated due to -Wfatal-errors.
BB-WAF: cc1plus: some warnings being treated as errors
BB-WAF: 
BB-WAF: Waf: Leaving directory `/home/autotest/build/tmp/binaries.build/ZubaxGNSS'
BB-WAF: Build failed
BB-WAF:  -> task in 'objs/AP_Compass' failed (exit status 1):
BB-WAF: 	{task 139682387568112: cxx AP_Compass.cpp -> AP_Compass.cpp.0.o}
BB-WAF:  (run with -v to display more information)
BB: Process failed ((11327, 256))
BB: Failed build of AP_Periph ZubaxGNSS latest

Issue details

Compilation appears to be broken with a bounds-sanity check.

Version
master (autotest server)

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine
[ X ] AP_Periph

Airframe type
Erp.

Hardware type
ZubaxGNSS

Logs
autotest build log: https://autotest.ardupilot.org/autotest-output.txt

@tridge @bugobliterator

[peterbarker]

This looks to be confusion between COMPASS_MAX_INSTANCES and HAL_COMPASS_MAX_SENSORS

I propose eliminating the use of HAL_COMPASS_MAX_SENSORS anywhere but when determining what COMPASS_MAX_INSTANCES should be set to (currently switches on whether we're doing an AP_Periph build, and the ZubaxGNSS AP_Periph isn't setting HAL_COMPASS_MAX_SENSORS to 1, so the two #defines are getting out of whack.